The present invention relates to optical filters and methods of making and using the optical filters. In addition, the present invention is directed to optical filters that include both reflective and lossy elements and methods of making and using such optical filters.
Optical filters, including, for example, optical mirrors and polarizers, can be used in a variety of devices to perform a variety of functions. In many of these optical filters, a portion of the light incident on the device is reflected or absorbed and another portion of the light is transmitted. For example, an IR filter reflects or absorbs IR light while passing visible light. As another example, a polarizer reflects light having one polarization over a particular wavelength band and transmits light having the orthogonal polarization.
A number of parameters can be used to describe the optical properties of optical filters. Transmission, T, (or absorption or reflection) of a wavelength or wavelength range to be absorbed or reflected can describe the amount of light leaking through an optical filter. Optical density is defined as xe2x88x92log10 T, and is particularly useful for comparing low transmission optical filters. In some applications, optical densities of 4 (corresponding to 0.01% transmission) or greater may be desired or required.
Another parameter is the sharpness of the band edge (e.g., the transition from a high transmission wavelength range to a low transmission wavelength range). For at least some applications, it may be desirable or required to have a change of 2.5 or more in optical density over a wavelength range of 5, 10, or 20 nm or less.
Absorptive optical bodies have been used for many of these applications. Typically, these optical bodies include an absorptive material deposited on or within a substrate (e.g., a glass or polymer substrate). Such absorptive optical bodies are often characterized by their percent absorptance, referring to the percentage of light that is absorbed in a single pass through the optical body. Unfortunately, absorptive materials typically do not have sharp, broadband absorption spectra. Generally, absorptive materials at least partially absorb light within the desired transmission wavelength range or polarization and are not very wavelength selective. For example, an absorptive IR or UV filter will also typically absorb a portion of the visible light. To increase the IR or UV absorption results in more absorption of visible light. For applications such as, for example, eyewear or window treatments, this phenomenon can limit how much IR or UV protection can be obtained based on the visibility requirements. In addition, the visible absorption is typically non-uniform, which can result in a colored appearance.
Broadband reflective optical bodies can be made using, for example, multilayer thin film techniques. The wavelength range of these optical bodies is typically a function of the range of thicknesses of the layers. The reflectivity of these optical bodies is improved by using more layers of each thickness. To obtain highly reflective, broadband optical bodies generally requires the controlled formation of a large number of thin films. As more layers are needed to obtain the desired optical parameters, the cost and difficulty in making the optical body increases.
Generally, the present invention relates to optical filters and their manufacture, as well as the use of the optical filters in optical devices, such as polarizers and mirrors. One embodiment is an optical filter that includes a first reflective multilayer film, a second reflective multilayer film, and a lossy element disposed between the first and second reflective multilayer films. The first and second multilayer reflective films each have an optical density, for unpolarized light, of at least 1 over the same wavelength range. The wavelength range has a width of at least 50 nm. The lossy element, in the absence of the first and second multilayer reflective films, causes the loss of, on average, at least 5% of light in that wavelength range. In addition, the present invention is related to the method of filtering light through this optical filter, as well as the other optical filters and articles described below.
Another embodiment is an optical filter that includes a first reflective element, a second reflective element, and a lossy element disposed between the first and second reflective elements. The second reflective element has a band edge within about 25 nm of a band edge of the first reflective element. The lossy element causes the loss of at least some light within the band edge of at least one of the first and second reflective elements. In some instances, the band edge of the lossy element is within at least 25 nm of a band edge of the first or second reflective elements. In some instances, two or more of the band edges overlap.
Yet another embodiment is an optical filter that includes a first reflective multilayer film, a second reflective multilayer film, and a lossy element disposed between the first and second reflective multilayer films. The first and second multilayer reflective films define a reflection wavelength range in which the first and second multilayer reflective films, in the absence of the lossy element, have an optical density of at least 1.5 for unpolarized light. The lossy element, in the absence of the first and second multilayer reflective films, has an optical density for unpolarized light of at least 0.1 over the reflection wavelength range.
Another embodiment is an optical filter that includes a first reflective multilayer film, a second reflective multilayer film, and a lossy element disposed between the first and second reflective multilayer films. The optical film has an optical density of at least 2 over a first wavelength range of at least 50 nm and has an optical density of no more than 0.3 over a second wavelength range of at least 50 nm. The lossy element, in the absence of the first and second multilayer reflective films, absorbs on average at least 5% of light in the first wavelength range.
A further embodiment is an article that includes at least one transparent substrate and an optical film disposed over a major surface of the substrate. The optical film has a first multilayer reflective film, a second multilayer reflective film, and a lossy element disposed between the first and second multilayer reflective films.
The above summary of the present invention is not intended to describe each disclosed embodiment or every implementation of the present invention. The Figures and the detailed description which follow exemplify several embodiments.